a) Field of the Invention
This invention relates to an interference microscope, constituting differential interference and phase-contrast types, in which the interference of light is utilized for observing the phase distribution of a sample.
b) Description of the Prior Art
Microscopes in which the interference of light is utilized for observing the phase distribution of a sample are available as phase-contrast and differential interference microscopes. An example of the arrangement of a conventional differential interference microscope is shown in FIG. 1. In this figure, light emitted from a source of light 1 is incident on a Wollaston prism 2, by which the light is split up into two beams vibrating in directions perpendicular to each other. These beams pass through a condenser lens 3 and irradiate a sample 5 on a slide glass 4. The beams are then transmitted through the sample 5 to enter an objective 6, and after having traversed the objective 6, is incident on a Wollaston prism 7. Since the combination of crystals of the Wollaston prism 7 is reversed to that of the Wollaston prism 2, the two beams entering the prism 7 are recombined to overlap. The overlapping beams are transmitted by a polarizing plate 8 oriented at an angle of 45.degree. with the directions of vibrations of these two incident beams, thereby interfering with each other on a CCD 9. Such an interference image represents the differential of phase distribution of the sample. Hence, according to the differential interference microscope, the situation can be observed even in a transparent sample.
Next, the arrangement of a conventional phase-contrast microscope is shown in FIG. 2. Light emitted from the light source 1 travels in such a manner that a part of the light emerges from an annular opening 10 composed of a ring-shaped slit, is converged by a condenser lens 11, and irradiates the sample 5 on the slide glass 4. The light is separated into transmitted light and diffracted light by an objective 12. The transmitted light is transmitted through a phase film 13 with a retardation of .lambda./4 and a transparent substrate 14 on which the phase film 13 is evaporated. The diffracted light, on the other hand, passes through the whole of the transparent substrate 14. Consequently, the transmitted light and the diffracted light interfere with each other on the CCD 9. The interference image available here represents a Fourier transform image of the sample 5. Thus, the phase-contrast microscope, like the differential interference microscope, allows the image to be observed even in a transparent sample.
In the foregoing microscopes, provisions have been made for producing the image with contrast which is visible with great ease. For example, the differential interference microscope of a certain type is equipped with such a mechanism that the Wollaston prism 7 can be laterally moved. Specifically, a user can turn a screw with his hand while viewing the image to move the Wollaston prism 7, thereby changing the phase difference between the two interfering beams of light. The user can thus set the optimum contrast. When the Wollaston prism 7 used as a wave recombining means is moved, however, its optical axis is also shifted. This adversely affects an imaging relationship in addition to a contrast. To suppress such behavior, it is necessary to improve the accuracy of straightness of a locus produced by the movement of the Wollaston prism 7. This, however, has been cumbersome and difficult.
As for the phase-contrast microscope, when the phase film 13 is set to have a phase difference of .lambda./4, the image is produced with the best contrast. However, since the contrast depends on the thickness of the phase film 13, the user cannot alter the contrast, when once it is set. Hence, the microscope has the disadvantage that even when the user intends to adjust the contrast of interference fringes in observation and examination, he cannot modify the contrast.